In the prior art, two basic approaches have been favored for providing filters which transmit all, or some portion, of the visible spectrum and reflect infrared radiation.
These two basic approaches are well known to those skilled in the art, accordingly they are discussed only briefly below.
A first approach involves depositing multilayer interference band pass filters comprising, entirely, dielectric layers.
Multilayer band pass filters may be in the form of multiple cavity or multiple half-wave band pass filters, which include a combination of alternating high and low refractive index dielectric layers, some of which have an optical thickness of about one quarter-wavelength at a particular wavelength, and some of which have an optical thickness of one-half of that wavelength. The wavelength at which the layers are one-quarter or one-half wavelength thick is generally designated the center wavelength, and generally corresponds to the frequency center of the wavelength range to be passed by the filter.
Multilayer band pass filters may also be in the form of combination of long and short wavelength pass filters, often termed edge filters. The combination generally includes at least one filter defining a short wavelength edge and designed to pass wavelengths longer than the short wavelength edge, and one filter defining a long wavelength edge and designed to pass all shorter wavelengths.
An advantage of all dielectric filters is that, because of the very low absorption possible in dielectric layers, transmission may be very high. Transmission may be limited essentially by the degree to which reflection can be reduced in the wavelength range to be passed by the filters.
A disadvantage of all-dielectric filters is that as many as twenty layers may be required to provide an adequately steep transition from a reflecting region, or stop region, to a transmission region. Fifty or more layers may be required to extend a stop region over a wide band of wavelengths. Extended stop regions are a particular problem for wavelengths longer than the wavelength region to be passed, as layers must be made increasingly thicker to block increasingly longer wavelengths. Further, complex layer configurations are required to prevent high order reflection bands of long wavelength blocking layers from appearing in the wavelength range to be passed.
A second approach to the deposition of multilayer band pass filters was proposed in a paper "Induced Transmission in Absorbing Films Applied to Band Pass Filter Design", Berning and Turner, J. Opt. Soc. Am. 74, 3, 230-239. In this approach, a metal layer, preferably a silver layer, is bounded on either side by multilayer dielectric reflecting layer systems comprising stacks of alternating high and low refractive index layers, each about one-quarter wavelength optical thickness at about the center of a wavelength range to be passed. On the long wavelength side of this range, the metal layer provides the desired blocking reflection. Such filters are generally termed induced transmission filters. Transmission is essentially "induced" through the metal layer by the quarter-wave multilayer stacks, which reduce reflection from the metal layer in the wavelength range to be passed.
Such filters were originally proposed as suitable for passing limited wavelength ranges, and were used, for example, as color filters in electro-optical systems. They are now used in a very simple form as low-emissivity (heat retaining) coatings for architectural glazing. In this simple form the metal layer is relatively thin, for example, about 10 nanometers (nm), and the dielectric stack is reduced to only one relatively high refractive index layer.
This simple form has a disadvantage that as the silver layer is relatively thin (for providing a pass region sufficiently wide to accommodate the visible spectrum) the filter is not effective in blocking near infrared wavelengths which make up a large proportion of the solar spectrum.
U.S. Pat. No. 3,682,528 (Apfel et al.) discloses a heat reflecting filter including two silver layers separated by a dielectric layer and bounded by dielectric layers. Such a filter is essentially two of the above described simple induced transmission filters in coherent optical contact. Each simple filter is generally designated a "period" by optical multilayer designers. In a paper "Graphics in Optical Coating Design", Applied Optics, 11, 6, 1303-12, Apfel teaches graphic design methods for designing filters including two, three, four, and more such periods, and illustrates their theoretical performance.
A "period" in such a filter may be conveniently designated by a shorthand notation DMD, wherein D represents a dielectric layer and M represents a metal layer. A two period filter would be designated DMDMD, a three period filter would be designated DMDMDMD, and so on. Those familiar with the thin film design art will be aware of the approximate thicknesses of the D and M layers in such filters.
In theory at least, a four period (four silver layer--DMDMDMDMD) induced transmission filter will provide, using only nine layers, a pass region extending over the visible spectrum, and a stop region extending from the near infrared region across essentially the entire infrared region. As discussed above, providing a similar stop region using dielectric layers would require more than fifty layers.
In practice, routinely producing even a two period, DMDMD type, broad band induced transmission filter is made difficult by the requirement for thin silver layers. Such thin layers provide at least two significant problems.
First, there is a problem of achieving and retaining optical properties of the silver film which are theoretically predictable. This problem has been addressed by depositing each silver layer on a nucleating layer of a metal such as nickel to provide the desired property. This is discussed in the above-referenced Apfel et al. patent. Once the layer is deposited, the silver is preferably protected by a barrier layer or the like before depositing a dielectric layer. This is not uncommon if layers of the filter are formed by sputter deposition. Such a barrier layer is discussed in U.S. Pat. No. 4,462,883 (Hart).
A second problem lies in controlling the pass band characteristics of the filter, particularly the transmission and reflection colors, even if the silver layer properties can be controlled. This problem is identified by Berning in a paper "Principles of Design of Architectural Coatings", Applied Optics, 22, 24, 41274141. The problem arises because when the silver layers have a thickness of about 11.0 nm or less, optimum dielectric layer thickness, particularly the spacer-layer thickness, is a sensitive function of silver layer thickness. This is explained in further detail below.
U.S. Pat. No. 5,183,700 discloses one alternative method of constructing a broad band pass filter wherein a single metal layer, preferably a single silver layer, provides long wavelength infrared reflection, and near-infrared reflection is augmented by means of high and low refractive index dielectric layers. In a preferred embodiment, the filter comprises five layers, and includes only one silver layer, the layer having a thickness of about 20 nm. The performance of the filter is comparable with, or even superior to, a DMDMD filter wherein the two metal layers (M) each have a thickness of about 10 nm.
It would be useful to provide a filter which has the characteristics of a prior art DMDMDMDMD (four metal layer) filter, but which preferably requires only two metal layers, preferably two silver layers each having a thickness greater than 12.5 nm.